Imminent implementation: Final Questions?

on returning to work from my week off on Monday, I will begin the replacement of OpenXchange with Zimbra.

I have successfully used imapsync on my test setup at home to get all mail from Cyrus IMAP accounts (which OX uses) to Zimbra.

However I have a few final questions before I launch it.

The MX thing is still vexing me slightly. I'll explain further. We have a Domain that has correctly setup MX records and have been running our own Mailserver on this which is able to receive and send email through an SME 7 server for the past year.
Therefore the MX record actually points at the SME mail/webserver machine and it is set in the SME interface to delegate the Mailserver role to the OX machine. This works fine, as soon as the email arrives with the SME it is immediately forwarded on to OX without anything being done to it.
I should simply be able to replace the OX server with the Zimbra machine and point the SME to delegate the Mailserver role to the Zimbra server instead shouldn't I?
Why I ask is because the MX record will obviously not point at the Zimbra server as it inside the Network and only accessible directly by the SME machine. We don't need it to be accessible outside the local network.
This has, as I say, worked fine for the last year with OX but is this OK for Zimbra?
I assume that as long as I put the SME as a Relay Server in the Global, MTA settings this will suffice. I just want to make sure that this will not have any adverse effect on the Zimbra setup because of the lack of an MX reference to it and will the 'Enable DNS Lookups' setting be needed on the Zimbra server if we are relaying the mail?

Secondly,
I have had the OX server setup with Spamassassin 2.64 using the sa-learn script to define mail as Spam, just like Zimbra does. Where should I be able to find this list on the OX machine (standard Spamassassin setup, SUSE SLES9) and can I then just import it straight into the same file in the Zimbra machine and Spamassassin use it for refernce just like that? Where would I find the destination spam file on the Zimbra machine?

And finally,

I still have to find a way to get all the Contacts and Calendar event. The Contacts are in LDAP and PostGreSQL and the Contacts in WebDAV. Has anyone any tips they could offer in getting these from OpenXchange or something similar?

Thanks for any help you can offer, this will be hopefully the last steps to allow me to get rid of OpenXchange (which is crap) and enter 'The Wonderful World of Zimbra'!